Mac only: If you're going to warble along with your favourite song, make sure you get the words right. Get Lyrical is a free app that automatically displays the lyrics of whatever song is playing in iTunes.
This tiny app grabs lyrics from lyrics.wikia.com and shows them in a resizable window as the song is playing. You can choose to have Get Lyrical display words to every song as it begins, or only the ones you select in your playlist. The app also recognises songs sung in Spanish. Don't like the lyrics that come with the song? Edit, change and save them right inside the app.